An arrhythmia is an abnormal heart rhythm. One example of an arrhythmia is a tachyarrythmia, wherein the heart beats at an abnormally fast rate. With atrial tachycardia, the atria of the heart beats abnormally fast; likewise, with ventricular tachycardia, the ventricles of the heart beat abnormally fast. Though often unpleasant for the patient, an atrial tachycardia is typically not fatal. However, some tachycardia, particularly ventricular tachycardia, can trigger ventricular fibrillation wherein the heart beats chaotically resulting in little or no net flow of blood from the heart to the brain and organs. Ventricular fibrillation, if not terminated, may be fatal. Hence, it is highly desirable to prevent or terminate arrhythmia, particularly ventricular tachycardia, with an appropriate therapy.
Implanted medical devices are sometimes used to detect and treat a patient's arrhythmia. As an example, an implanted medical device may include a defibrillator which applies electrical therapy to a patient's heart upon detecting an atrial fibrillation. Cardioverters or defibrillators discharge relatively high energetic electrical shocks across cardiac tissue to arrest life-threatening ventricular fibrillation detected by the implanted medical device. Defibrillation shocks, while highly effective at arresting the fibrillation, may cause considerable patient discomfort and should therefore only be applied if they are really necessary. Other therapy of tachycardia or fibrillation includes drug administration, wherein the drugs are often highly specific with regard to the type of tachycardia or fibrillation.
In order to apply the most appropriate therapy to the patient suffering an arrhythmia, it is necessary to know the particular type of an arrhythmia occurring with a patient. For example, if high ventricular rate is accompanied by high atrial rate, then a decision needs to be made to distinguish SVT (Supra Ventricular Tachycardia) from VT (Ventricular Tachycardia). Although other criteria—such as sudden onset, stability, AV consistency, QRS morphology, etc. of a cardiac signal—can be used to facilitate the classification, the sensitivity and specificity of these methods are not sufficiently high. Further, these methods require substantial time to make a distinction between different arrhythmia types.
It would therefore be useful to have methods and devices for reliably and quickly determining the type of arrhythmia, with high sensitivity and specificity, so that the most appropriate therapy might be provided to the patient shortly after detection of the arrhythmia.
US Patent Appl'n. Publ'n. 2004/0267321 A1 describes an implantable cardiac stimulation device which is configured to automatically monitor the effects of antiarrhythmic drugs on cardiac electrical signals within a patient to verify the efficacy of the drugs taken. The device utilizes atrial and ventricular sensing circuits to sense cardiac electric signals to determine whether a rhythm is physiologic or pathologic. The sensed signals are then processed in order to determine the presence of an arrhythmia. Timing intervals between sensed events (e.g., P-waves, R-waves, and depolarization signals associated with fibrillation, which are sometimes referred to as “F-waves” or “FIB-waves”) are classified by a microcontroller by comparing them to a predefined rate zone limit and various other characteristics (e.g., sudden onset, stability, physiologic sensors, morphology, etc.). The device further includes an antiarrhythmic drug efficacy monitoring unit for automatically monitoring the efficacy of antiarrhythmic drugs prescribed to the patient. The monitoring unit therefore includes a cardiac signal analysis unit for analyzing the patient's cardiac signal to verify the efficacy of the prescribed drugs, and a warning signal generation unit for generating a warning signal alerting the patient or physician to possible drug efficacy problems. The monitoring unit also includes a drug pump control unit for automatically controlling an optional implantable drug pump to compensate, if necessary, for drug efficacy problems. For example, if an initial dosage of an antiarrhythmic drug is not adequately effective, the drug pump may be controlled to increase the dosage. Additionally, the device includes a monitoring unit including a control parameter adjustment unit for automatically adjusting pacing control parameters used by the implanted device to compensate for drug efficacy problems. For example, if the prescribed antiarrhythmic drug is not adequately effective, overdrive pacing control parameters can be adjusted to increase the aggressiveness of overdrive pacing.
A disadvantage of the device of US 2004/0267321 A1 is that it is slow to determine arrhythmia, and stabilization of antiarrhythmic drug delivery also takes time.
U.S. Pat. No. 6,968,226 B2 is directed to an implanted device which automatically prevents and/or terminates an atrial arrhythmia in a patient's heart using pacing and/or pharmaceutical therapies. The device measures at least one electrocardiogram characteristic indicative of an atrial arrhythmia, thereby detecting an atrial arrhythmia of the heart, and thereafter transmits a warning signal to the patient. Instead of, or in addition to, the traditional electrical cardioversion therapy, the patient chooses a pharmaceutical therapy via an available drug delivery arrangement, for example via an external drug delivery arrangement (e.g. intravenously, orally, transdermally, intramuscularly, orally, inhalationally, among others) to terminate the atrial fibrillation. While delivering the drug therapy or shortly thereafter, the implanted medical device measures at least one additional Q-T interval and denominates it as the drug therapy Q-T interval. Drug therapy is terminated if a drug therapy Q-T interval is measured to be greater than a drug therapy Q-T interval threshold. For example, drug therapy is stopped if the Q-T interval after initiation of drug therapy is measured to exceed 500 ms Like the system of the aforementioned 2004/0267321 A1, this system is also slow to determine arrhythmic type, and involves a long term therapy of arrhythmia as well.